Hysteresis synchronous electric motor



4 Sheets-Sheet 1 A. BEKEY mwx. Nu

HYSTERESIS SYNCHRONOUS ELECTRIC MOTOR V6 Mad' #arf/6%;

. N mmm mmm www @N EN QS \mm. Q w. m91 mwN June 5, 1962 Filed June 22. 1959 June 5, 1962 A. BEKEY HYSTERESIS sYNCHRoNoUs ELECTRIC MOTOR 4 Sheets-Sheet 2 Filed June 22. 1959 June 5, 1962 A. BEKEY 3,038,092

HYsTEREsIs sYNcHRoNoUs ELECTRIC Mo'roR June 5, 1962 A. BEKEY HysTEREsIs sY1\1cHRoNoUsA ELECTRIC MOTOR 4 Sheets-Sheet 4 Filed June 22. 1959 .faq

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' d352/wey@ United States Patent Ofice 3,038,092 Patented June 5, 1962 3,038,092 HYSTERESHS SYN CHRNUS ELECTRIC MOTOR Andrew Bekey, Los Angeles, Calif., assignor to Genisco Incorporated, Los Angeles, Calif., a corporation of California Filed June 22, 1959, Ser. No. 821,767 12 Claims. (Cl. 310-162) The present invention relates to alternating current synchronous motors of the hysteresis type, and it relates more particularly to an improved rotor construction for such a motor.

The rotor of the hysteresis motor is usually cylindrical in shape, and it is usually made up of a block of magnetic material of relatively high coercivity. Alternately, the rotor may be composed of a stack of punched laminations of high coercivity magnetic material. The rotor is mounted on the drive shaft of the motor, and it is supported by the drive shaft for rotation in the bore of the motor stator.

The stator of the hysteresis motor creates a rotating magnetic field which crosses the annular air gap between the stator and the rotor and which then enters the rotor. The rotating stator magnetic field exhibits one or more pairs of radial magnetic polarities depending upon the number of stator poles in the motor. These magnetic polarities of the rotating stator flux are equiangularly disposed about the rotational axis of the rotor, and they rotate about that axis.

A torque is created in the rotor of the hysteresis motor by hysteresis action in the active high coercivity magnetic rotor material. When the motor is first started, the rotating stator field induces magnetic poles in the rotor, and these poles slip with respect to the rotor until the rotor cornes up to synchronous speed. At synchronous speed the rotor material becomes permanently magnetized, and it exhibits a plurality of radial magnetic poles at its peripheral surface. These radial magnetic poles of the rotor are equiangularly displaced from one another, and they correspond in number and in angular disposition to the radial poles exhibited by the rotating stator field.

In a hysteresis motor a stator flux is rotated about a rotor of relatively high coercivity active magnetic material, the poles induced in the rotor react with the rotating stator field. This reaction is such that a torque is exerted on the rotor. However, this torque is created only if an angular displacement exists between the poles of the rotating stator field and the magnetic poles induced in the rotor. This angular displacement is due to the hysteresis losses in the rotor as expressed by the area of the B/H hysteresis loop of the magnetic material forming the rotor. The torque developed is directly dependent upon such hysteresis losses, and this torque may be expressed by the following equation:

where:

T is the torque exerted on the rotor P is the number of pairs of poles induced in the rotor V is the volume of the rotor Wh is the area of the B/H hysteresis loop of the active magnetic material in the rotor.

It is usual in the prior art hysteresis motor to form at least a portion of the rotor out of magnetic material of relatively high coercivity, as mentioned above. This active portion of the rotor in the prior art hysteresis motors usually has an annular sleeve-like configuration. The annular active portion of the prior art rotor is usually concentrically mounted on a hub of magnetically inactive material, and the hub is keyed to the motor drive shaft. The resulting rotor assemblyis supported by the drive shaft for rotation in the bore of the stator, as mentioned above.

An examination of Equation 1 will reveal that the torque per unit volume in a hysteresis motor having a particular number of pole pairs depends directly upon the area of the hysteresis loop of the active hysteresis portion of the rotor. This area, in turn, depends upon the magnetic condition under which the hysteresis material in the active magnetic portion of the rotor is operating.

As the magnetizing force of the stator of the hysteresis motor is increased, the hysteresis loss in the rotor will increase up to saturation of the rotor material, with resulting increase in the torque exerted on the rotor. However, practical `design requirements dictate a particular maximum magnetizing force for a particular motor, and it is an important object of the present invention to provide a hysteresis rotor for such a motor in which maximum and most efficient use is made of the available magnetizing force.

For maximum available magnetizing force in a particular motor, the active portion of the rotor must be designed to be driven to a particular optimum magnetic density. This magnetic density should, for example, approach the saturating limit of the particular rotor but should not extend into the saturating region. If the magnetic density in the rotor is too high, the magnetic material of the rotor will be driven into its saturated region with resulting waste of the available magnetizing force insofar as the production of a hysteresis torque is concerned. On the other hand, if the magnetic density in the rotor is -too low, the resulting hysteresis loop for the available magnetizing force has an area which is less than optimum with a resulting loss in the optimum production of the hysteresis torque.

It follows from the above considerations that in a given hysteresis motor with a given amount of magnetizing force available from the rotating stator flux, for maximum efficiency, the rotor must be constructed to be driven to a particular maximum magnetic flux density for optimum hysteresis loss and maximum hysteresis torque. It also follows that under conditions where the available magnetizing force varies around the rotor of the motor, the rotor must be constructed so that full use is made of the available magnetizing force at all points in the rotor material.

The prior art annular rotors exhibit a flux path of constant cross-sectional area for the magnetic forces from the stator which exist around the annular air gap between the rotor and the stator. As will be explained in more detail subsequently, even if the stator flux has a uniform distribution in the annular air gap, the provision of a constant cross-section for the flux in the rotor leads 4to a condition in which the iiux density in the rotor changes through Wide ranges at different angular positions around the rotor when the motor is operating at synchronous speeds. This results to a condition in which portions of the rotor are driven by the magnet@ motive forces into saturation regions for a waste of the magnetomotive forces insofar as creating hysteresis torque is concerned; or portions of the rotor are driven in hysteresis loops of less than optimum area by the magnetizing forces so that less than optimum hysteresis torques are produced by the later portions on the rotor.

An important feature of the present invention is the provision of a hysteresis motor whose rotor is constructed to exhibit a uniform optimum magnetic flux density throughout its entire cross-sectional area, despite variations in the total flux carried by different portions of the rotor. The rotor of the improved hysteresis motor of the invention may be constructed to exhibit, therefore, a B/ H hysteresis loop of optimum area uniformly throughout its entire cross-section for the optimum and most efficient production of hysteresis torque.

The improved rotor construction of the hysteresis motor of the invention enables maximum and most eicient use to be made of the magnetomotive forces available in the motor. This, in turn, results in a maximum torque output for a given magnetizing current in the stator of a particular motor. The improved hysteresis motor of the invention is, therefore, capable of producing greater torques at Abetter eciency as compared with a prior art hysteresis motor of similar parameters. Moreover, the hysteresis motor of the present invention is capable of producing increased hysteresis torque `with reduced rotor volume and weight, as compared with a corresponding prior art motor.

The above and other features of the invention which are believed to be new are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood by a consideration of the following specification when taken in conjunction with the accompanying drawings.

In the drawings:

FIGURE 1 is a representation of a family of B/H hysteresis loop curves for a particular magnetic body; these curves showing the effect on the hysteresis loop, and on its circumscribed area, of the introduction of different maximum magnetomotive forces to the magnetic body;

FIGURE 2 is a cross-sectional schematic view of a prior art type `of hysteresis motor including a rotor having an annular active magnetic portion of substantially uniform cross-sectional area, and in which the rotating stator field is represented in one instantaneous position by a pair of stator poles constructed to provide essentially uniform flux distribution in the annular air gap of the motor;

FIGURE 3 is a schematic representation of the flux Ldistribution in any one of the four quadrants of the rotor of FIGURE 2, the lux density of the illustrated rotor quadrant being shown as increasing from a minimum at one end to a maximum at the `other end despite the uniform flux distribution in the annular air gap;

FIGURE 4 is a diagrammatic representation which is intended to illustrate more clearly and on a quantitative basis the varying linx density in the illustrated rotor quadrant of FIGURE 3;

FIGURE 5 is a graph illustrating the linear relationship between the increasing flux density in the rotor quadrant of FIGURE 3 for increasing angular displacements from a reference angular position in the rotor, and in the presence of a uniform flux distribution in the annular air gap ofthe motor;

FIGURE 6 is a graph illustrating a particular nonuniform air gap flux distribution, in which the flux in the annular air gap of the motor drops abruptly from a maximum value to a value yof approximately 50% of maximum at points in the annular air gap;

FIGURE 7 is a diagrammatic representation of a rotor quadrant, similar to the representation of FIGURE 4, but in which weighting factors are included for taking care of `changes in the flux in the air gap of the type illustrated in the graph of FIGURE 6;

FIGURE 8 is a curve illustrating a sinusoidal relationship between the increasing flux distribution in the rotor quadrant of FIGURE 3 for angular displacements in the rotor from a reference rotor position and in the presence of a sinusoidally distributed magnetic flux in the annular air gap;

FIGURE 9 is a developed schematic view of a hysteresis motor incorporating a prior art rotor of annular conguration and essentially constant cross-sectional area, the instantaneous position of the rotating stator flux being illustrated by physical magnetic poles in this view, and these serving to illustrate the flux distribution in the illustrated prior art rotor;

FIGURE 10 is a developed schematic view of a hysteresis motor, similar to the view of FIGURE 9, but having a rotor shaped in accordance with the invention to exhibit a uniform magnetic density throughout its entire cross-sectional area;

FIGURE 1l is a sectional view in schematic form of a four pole hysteresis motor, in which the rotating stator field is illustrated in an instantaneous position by physical magnetic poles, and which includes a hysteresis rotor shaped in accordance with the present invention to exhibit uniform magnetic density throughout its cross-sectional area;

FIGURE 12 is a cross-sectional view of a typical stamped rotor lamination for a two pole ihysteresis motor, the illustrated rotor lamination being formed in accordance with the invention to have a varying section which is calculated with respect to the llux `distribution the air gap of the motor so that the rotor may exhibit a uniform magnetic density in certain regions;

FIGURE 13 is a cross-sectional view of a two pole hysteresis motor constructed in accordance with the invention and which includes a rotor formed to have a configuration similar to the configuration of the rotor lamination of FIGURE l2; and

FIGURE 14 is a longitudinal sectional view of the rotor assembly of the motor of FIGURE 13.

The Ihysteresis loops of FIGURE l, as mentioned, represent a family of B/H hysteresis curves for a partcular magnetic material. The magnetic material selected, by way of example, was 17% cobalt steel. 'I'he particular test sample was in the form of a bar of `one-half inch by three-eighths inch; annealed; RC33; hardening temperature 1750" F.; `oil quenched. The illustrated loops show the eiect of different magnetomotive forces on the area of the hysteresis loops of the particular material'.

It will be observed from the curves of FIGURE l that the area of the hysteresis loops diminish at a greater rate than a corresponding diminishing magnetomotive.

force, so that the ratio between the hysteresis loop area and magnetomotive `force is not constant. It will also be observed that an optimum area for a particular magnetomotive force occurs, when the magnetic material is driven substantially to, but not into, its saturation region. It follows, therefore, that for optimum hysteresis torque, which depends on optimum area of the hysteresis loop, the magnetic material of the rotor should be constructed to exhibit the particular magnetic densities required to provide the desired optimum hysteresis loop in the presence of ydifferent magnetomotive forces which normally exist around the rotor.

In the prior art hysteresis motor, if the dimensions of the active hysteresis portion of the rotor are chosen for an optimum area of the hysteresis loop for the available magnetomotive forces at a portion of maximum magnetic density in the rotor, then the magnetomotive forces in other portions in the rotor produce hysteresis loops which are less than optimum area. The active hysteresis portion of the rotor in a prior art hysteresis motor exhibits, therefore, a whole family of hysteresis loops, similar to the family shown in FIGURE 1. It follows then, that if some of the hysteresis loops in the prior art rotor exhibit optimum area for the available magnetomotive forces, others will exhibit less than optimum area, or others will exhibit saturation effects which diminish the efficient use of the existing magnetomotive forces.

As noted above, the present invention provides an improved rotor construction for a hysteresis motor. In the improved rotor of the present invention, the varying magnetomotive forces which normally exist in the rotor produce a uniform magnetic density in the rotor. This uniform magnetic density is such, for example, that all portions of the active magnetic part of the rotor are driven substantially to the same saturation region for optimum area of the corresponding hysteresis loop. This results in optimum rotor torque and in the most eicient use of weight and volume in the magnetic material forming the active portion of the rotor.

For purposes of explaining. the principles underlying the present invention, a prior art type of hysteresis motor will first be considered. Such a prior art motor is illustrated in the cross-sectional, somewhat diagrammatic representation of FIGURE 2. The prior art hysteresis motor of FIGURE 2 is assumed to be of a two pole type, as mentioned above. As also mentioned, it is assumed that the rotating stator iiux appears with essentially uniform distribution in the annular air gap of the motor.

The instantaneous position of the rotor of the stator ux is illustrated schematically in FIGURE 2 as a physical magnetic pole circumscribing the top semi-circular portion of the rotor and a south magnetic pole 11 circumscribing the bottom of the semi-circular portion of the rotor. The -pole I0 is illustrated as exhibiting north polarity in a radial direction with respect to the rotor, and the pole 11 is exhibited as exhibiting south polarity in the radial direction. These north and south radial polarities exhibited by the stator poles rotate about the axis of rotation of the rotor, and they provide radial iiux lines of substantially uniform distribution in the annular air gap 14 of the motor.

The prior art motor of FIGURE 2 includes a rotor assembly 12 which is mounted for rotation in the bore of the stator 10 about the axis of rotation referred to above. The annular air gap 14 separates the rotor from the stator. The rotor assembly includes an annular active hysteresis portion 16, and this portion is composed of a magnetic material of relatively high coercivity, such as 17% cobalt steel. That particular material has a relatively high coercivity, and it is produced, for example, by the Simonds Saw and Steel Company of Lockport, New York. The material is designated by that company as their No. 81 magnet steel.

The annular active hysteresis magnetic portion 16 of the prior art rotor assembly 12 is concentrically mounted on an annular hub 18. The hu-b 13 is usually composed of non-magnetic material, such as brass, and it is mounted on the drive shaft 2t) of the motor. The drive shaft 20 is rotatably supported in appropriate bearings for rotation about the axis of rotation of the rotor. The portions 18 and 16 of the prior art rotor assembly form a unitary structure with the drive shaft 20, and these portions are supported by the drive shaft for rotation in the bore of the stator 10.

For purposes of the following discussion, the active hysteresis magnetic portion 16 of the prior art rotor of FIGURE 2 will be assumed to have infinite permeability, and it will also be assumed that there is no leakage in the motor. The diagrammatic representation of FIGURE 3 shows the flux distribution in the first quadrant of the rotor 16 of the prior vart hysteresis motor of FIGURE 2. The ux distribution is symmetrical in the four quadrants of the rotor, so that only the first quardant need be considered, and for that reason only the first quadrant is shown in FIGURE 3.

As shown in FIGURE 3, which assumes that the stator pole 10 exhibits a north polarity at the particular illustrated instant, then the lines of magnetic force will iiow from the stator pole 111 and will enter the annular air gap 14 in a radial direction. As the flux lines enter the rotor from the air gap, they follow the relatively high permeability path oifered by the active portion 16 of the rotor,

and they extend circumferentially around the annular configuration of that portion into the fourth quadrant of the rotor (not shown). The ux lines leave the fourth quadrant of the rotor and extend into the stator pole 11 which, at the illustrated instant, exhibits a south pole in the illustrated position in FIGURE 2.

The varying iiux density in the fourth quadrant of the rotor is the same as the varying density of flux lines illustrated in FIGURE 3 for the iirst quadrant, except that the varying concentration in the fourth quadrant is in the opposite sense to that illustrated in the iirst quadrant. A similar varying flux density exists in the third and second quadrants of the rotor. It will be observed from the schematic illustration of FIGURE 3 that, even in the presence of a uniform ilux distribution in the annular air gap i4, the iiux density in the annular active portion 16 of the prior art rotor is not uniform. Instead, the flux density increases from the top of the quadrant in FIGURE 3 to the bottom. This change in flux density is shown on a more quantitative basis in the schematic illustration of FIGURE 4.

In the representation of FIGURE 4, the illustrated rotor quadrant of FIGURE 3 is divided into nine segments of 10 each, and the quadrant is also divided into 9 concentric sections. That is, the prior art active magnetic hysteresis portion 16 of the rotor is considered in FIGURE 4 to be composed of 9 concentrically arranged cylinders, with the cylinders being in contact with one another. The flux in the air gap 14 of the prior art hysteresis motor of FIGURE 2 is divided in 9 parts in the diagrammatic representation of FIGURE 4, with each part being represented by a single flux line. These air gap ilux lines, as mentioned above, extend in a radial direction through the air gap, and they are disposed with essentially uniform distribution in the air gap.

As illustrated in FIGURE 4, in the 01-l0 segment the flux density is only one line per unit area, while in the SIF- segment the flux density is 9 lines per unit area. Each segment in the quadrant, progressing in a clockwise direction receives one ilux line entering from the annular air gap 14, and each segment further carries in respective ones of its concentrical cylindrical sections all the other flux lines which enter the preceding segments. Since ail the segments in this prior art rotor have the same number of concentric cylinder sections and all have the same dimensions, it follows that the flux density in any segment is proportional to the number of illustrated tiux lines contained in that particular segment. The flux intensity in the illustrated quadrant of FIGURE 4 is represented by the curve which is referenced to the arcuate peripheral line of the rotor, that line forming a base for the curve It will be observed that the curve ,8 rises from Zero at i to a maximum at 90.

For the case of a uniform radial flux distribution in the annular air gap 14, as is realized in the prior nrt motor of FIGURE 2, the flux density in the first rotor quadrant increases from the 0 angular position to the 90 angular position in a direct proportional relationship, as illustrated by the straight line graph of FIGURE 5. However, the iiux distribution in the annular air gap of a hysteresis motor is usually non-uniform, especially with respect to the peripheral surface of the rotor. The flux distribution and density in the prior art rotor quadrant for a nonuniform flux distribution in the annular air gap can be determined by modifying the procedure described above. In such a modification, a Weighting factor is assigned to each ux line entering each segment of the quadrant under consideration. This weighting factor corresponds to the value of the distribution function of the field strength.

A non-uniform ux density in the annular air gap 14 over the first rotor quadrant is illustrated, f-or example, in FIGURE 6. In the illustrated case, the air gap iiux density is assumed to drop to one-half of its original value atan angular displacement of 45 in the quadrant. FIG- URE 7, which is a diagrammatic representation similar aosaooa to the representation of FIGURE 4, then illustrates how the weighting factor is asigned to each magnetic ux line asumed to enter the rotor in the quadrant under consideran tion.

The following general equation may be used to calculate the iiux densities in the prior yart active rotor portion of uniform cross-section:

where i is the th segment in the quadrant K is the area of the segment increments (assuming unity depths) W1 is the weighting function assigned to the flux line entering the ith segment to correspond to the iiux density in the air gap at that segment (normalized to a maximum value of one) n is the magnetic density at the nth segment M is the normalized strength of the magnetic field.

If the increments taken are small enough, Equation 2 then becomes:

ispirati/.orda (a) where is the flux densit at an angular position on the rotor displaced 6 from a reference.

For a sinusoidal eld distribution in the annular air gap, such that f()=cos 0; 0090 Then FLIrM. @0s ad@ (4) :K M. sin 0 (5) The relationship existing in Equation 5 is shown graphically in the curve of FIGURE 8. This latter curve represents the flux density `at an angular position in a rotor quadrant for a sinusoidal flux distribution in the annular air gap. This curve, of course, refers to a prior art rotor section which includes an `annular-shaped active hysteresis portion of uniform cross-sectional area. Accordingly, if a sinusoidal flux distribution exists in the annular air gap 14 `of the prior art hysteresis motor of FIGURE 2, the magnetic density in the active annular portion 16 of the Vrotor 12 of uniform cross-sectional area changes at a rate which is a function of the sine of the angular displace ments about the rotor, when such displacements are considered as a base for the curve, as shown in FIGURE 8.

It follows, therefore, that uniform magnetic density can be achieved in the rotor of a hysteresis motor by shaping the cross-sectional area of the active portion of the rotor in Aaccordance with a complementary function of the yannular air gap ux distribution at the outer peripheral surface of the rotor. While the sinusoidal air gap flux distribution has a general application, the square wave type of ux distribution such as referred to in conjunction with FIGURE 6 is often encountered. This latter type of distribution may be produced, for example, by transistorized power supplies.

The developed schematic view of FIGURE 9 of :a two pole prior art type of hysteresis motor is illustrated as embodying a pair of stator poles 30 and 32 which, as mentioned, represent an instantaneous position of two opposite polarities in the rotating stator flux. The annular active portion of the rotor is shown by the developed section 34, and this section is shown as having a uniform cross-section in accordance with the prior art practice.

The line O.D. in FIGURE 9 marks the outer cylindrical surface of the prior art rotor 34 which defines one boundlary of an annular `air gap 36, and the line I D. corresponds to the inner limit of the active magnetic material in the prior art rotor. The dotted lines in FIGURE 9 represent the lines of magnetic iiuxpassing from the nor-th stator polarity through the air gap into the rotor and back to the south stator polarity. The schematic illustration of FIG- URE 9 shows that those parts `of the rotor section 34 which are situated directly under the pole centers of the rotating stator magnetic flux have the lowest magnetic flux density, while those parts of the rotor section situated midway between the pole centers have the highest tlux density.

In FiGURE 10, the rotor section 34 `of FIGURE 9 has been replaced by a rotor 38. The rotor 38 is conistruoted in accordance with the concepts of the invention, and it diifers from the rotor 34 of FIGURE 9 in that its I.D. line, instead of being a straight line, follows a predetermined curve. The curvature of the LD. line of the rotor section 33 in FIGURE l() should lbe calculated in accordance with Equation 5, assuming a sinusoidal flux distribution in the air gap 36.

The shape of the rotor section 38 is such that `all areas Within the active portion of the rotor section operate at uniform magnetic density. As noted above, for optimum hysteresis torque development, the rotor 38 is preferably constructed so that all areas of its active portion are driven near to, but not into, their saturation region.

The representation of FIGURE l1 is somewhat similar to that of FIGURE l0, with the exception that the motor is represented in FIGURE ll in a circular section view, rather than in a developed View. Moreover, FIGURE ll illustrates a four pole motor in which the stator poles 40 and `412 are added to the previous stator poles 30 and 32. The stator poles are positioned at right angles to one 1another. It is to be remembered that the representations of FIGURES 9-11 1are diagrammatic and that the poles 30, 32 :and 40, `42 actually represent instantaneous radial polarity centers exhibited by the rotating stator ux.

The inner contours of the active rotor portion 38 in FIGURE 1l `are calculated to diminish in each quadrant from a maximum to a minimum cross-sectional area and in accordance with the sine of the angle from the circular O.D. line, when that line is taken as an abseissa. As illustrated, the configuration of the rotor quadrant between each pair of poles is the same. In each instance, the cross-section of the active material in the rotor approaches zero under the center of each pole, and the cross-section of the active matelial rises in the illustrated `configuration to a maximum between each pair of poles.

The active rotor section 38 is illustrated in FIGURE l1 as being mounted on a hub 44 of non-magnetic material. The hub, in turn, is mounted on a drive shaft 46. As in the previous embodiments, the drive shaft 46 is rotatably mounted on appropriate bearings, and the drive shaft supports the rotor assembly for rotation in the bore of the stator.

For operation at sub-synchronous speeds, the illustrated zero cross-sectional area portions of the active rotor section in FIGURE ll should have a nite area. For example, this crosssectional area `at the minimum positions should preferably be such that the magnetic ilux density therein is less than 100,000 lines per square inch.

As a further consequence of the shape of the active rotor section 38 in FIGURE ll, the magnetic symmetry of the usual prior art hysteresis rotor is disturbed and a pronounced polar saliency is achieved. This saliency also assists in providing additional torque, and it also helps to reduce rotor hunting in the motor. Moreover, this saliency effect in the rotor of the invention provides for synchronous lock-in of the rotor to occur at a particular pre-established angular relationship between the drive shaft and the rotating stator flux. This means that the rotor of the invention will lock-in with the rotating stator ilux with the required angular inter-relationship between the rotor and stator flux so that the rotor crosssectional areas are properly disposed to provide the desired uniform flux densities in the rotor.

It should be pointed out that the improved rotor construction of the invention, as shown schematically in FIGURES and 11 may be carried out without disturbing the outer cylindrical surface of the rotor in any way. The rotor of the illustrated embodiment of the invention, therefore, preserves the cylindrical shape of the prior art rotor to achieve the desired salience eifects and without disturbing the dimensions or conguration of the annular air gap between the rotor and the stator, or disturbing its uniform width. However, ift is clear that, when so desired, the required uniform magnetic density in the rotor can be achieved by shaping the outer surface of the rotor.

A constant magnetic density rotor incorporating the concepts of the invention may be manufactured, for example, by casting the rotor in a pattern built to the required configuration. Alternately, the rotor may be formed by stamping out rotor laminations from appropriate sheet metal, which laminations are later stacked to form the rotor. When such stampings are used, the die utilized to stamp out the internal diameter configuration of the rotor laminations can also be used to stamp out laminations of non-magnetic material to form the mating hub 44 which lits between the active rotor section 38 yand the shaft 46.

It is apparent from the foregoing description, that hysteresis rotors can be constructed in accordance with the concepts of the invention for uniform magnetic density, regardless of the iux distribution in the air gap, so long as the flux distribution in the `air gap can be calculated. As most hysteresis motors operate with a near sinusoidal liux distribution in the air gap, the following description will be directed to a constructed embodiment of the invention in which the air gap exhibits a substantially sinusoidal distribution of air gap flux.

The constructed embodiment of the invention has been tested, and that embodiment demonstrated conclusively that a hysteresis rotor constructed in accordance with the present invention to have a uniform r'iux density throughout its entire crossasectional area produces more torque for less weight, as compared with la prior art rotor of constant cross-sectional area, when equal magnetizing currents are used. By the same token, the rotor of the present invention in its constructed embodiment required less rnagnetizing current to provide a particular torque than the magnetizing current required by the corresponding prior art motor to cause its uniform sectional area rotor to produce the same torque.

To test the performance of a motor constructed in accordance with the invention, two rotors and a single stator were constructed. Both rotors were formed of the same magnetic material, and both were heat treated for the same length of time and in the same process. Both rotors were provided with the same outer diameters and with the same length. However, one of the rotors was made to have an annular active magnetic portion in accordance with prior art practice and with a uniform cross-sectional `area. The other rotor, however, was provided with yan active portion having a cross-sectional area which varied from point to point along a calculated curvature and in accordance with the teaching of the invention.

The stator' of the constructed test motor was a two pole, 60 c.p.s., 3 phase type. The stator was provided with -a bore of 2.925 inches and a length of active material of 1 inch. The cross-sectional configuration of the rotor constructed in accordance with the invention was as illustrated in FIGURE 12. The active material in both the rotors was a 17% cobalt steel.

Using the characteristic curves for the rotor material, and assuming a sinusoidal ilux distribution in the air gap with a maximum liux density of 75 kilogauss in the ro- 10 tor, the following Values and characteristics were calculated, and the accuracy of the calculations was conclusively demonstrated during the tests:

rI'he sectional views of one embodiment of the invention shown in FIGURES 13 and 14 illustrate that embodiment as including a stator 100. The stator has a plurality of radially disposed slots i102', and these slots extend around the stator at equiangular positions. The slots 102 are of the open ended type, and the open ends of the slots extend into the bore of the stator through its inner annular surface. 'Ihre stator winding is supported Within the slots 102. In the illustrated embodiment, this winding produces a rotating magnetic flux in lthe stator bore which exhibits a single pair of radial polarity centers.

A rotor assembly is mounted for rotation in the bore of the stator 100. This assembly includes yan active magnetic rotor section 104 which is shaped in accordance with the configuration of FIGURE 12. This active magnetic section of the rotor is formed of a magnetic material of relatively high coercivity. As mentioned above, an appropliate material for that purpose is 17% cobalt steel. As also mentioned above, the active magnetic section 104 of the rotor may be formed from an appropriate casting of the selected magnetic material, or by a stack of punched laminations of the material.

The outer peripheral surface of the magnetic rotor portion 104 has 'a cylindrical configuration, and this surface defines an annular air gap 106 with the inner annular surface of the stator 100. This air gap, because of the cylindrical configuration of the peripheral surface of the rotor, has a desired uniform width. The inner annular surface of the rotor is shaped, as mentioned, in accordance with the curve of FIGURE 12, so that the cross-sectional area of the active section 104 will be such that uniform magnetic density exists in that section.

The active magnetic rotor section 104 is supported on a hub 110. The hub y is at least partially formed of non-magnetic material, and it has an outer surface which coincides with the inner surface of the active magnetic section 104. This permits the active magnetic section 104 to be concentrically mounted on the hub 110. The hub 110, in turn, is mounted on a drive shaft 112. 'Ihe drive shaft 112 is rotatably mounted in appropriate bearings, and it serves to support the rotor assembly for rotation in the bore of the stator i100.

In accordance with the concepts and principles described above, the crea-tion of a rotating magnetic field in the annular air gap 106 produces essentially uniform magnetic flux density throughout the non-uniform crosssectional part of the active magnetic section 104 of the rotor. This flux density is such that an optimum area hysteresis loop is produced for the production of optimum hysteresis torque.

The invention provides, therefore, an improved alternating current synchronous motor of the hysteresis type. The improved motor of the present invention is such that optimum use is made in the rotor of the available magnetizing forces. The net result is that motors constructed in accordance with the present invention are capable of producing greater hysteresis torques than the prior art hysteresis motors, and of accomplishing this at reduced motor sizes and with reduced volume and weight of active material in the rotor itself.

I claim:

1. In alternating current apparatus of the hysteresis type, the combination of: a stator structure having a bore and including a stator winding for producing a rotating magnetic flux in said bore, a rotor mounted for rotation within the bore of the stator in concentric relationship with the stator and defining an annular air gap with the stator, said rotating magnetic flux having a particular distri-bution in said air gap and exhibiting a predetermined number of angularly displaced radial polarity centers, and an active component included in the rotor having an annular configuration and of a varying cross-sectional area, said active rotor component being composed of hysteresis magnetic material of a particular coercivity and presenting a path of essentially uniform flux density to the rotating flux in said stato-r bore during synchronous operation of the motor.

2. In an alternating current synchronous motor of the hysteresis type, the combination of: a stator structure having a bore and including a stator winding for producing a rotating magnetic flux in said bore, a rotor rotatably mounted within the bore of the stator in concentric relationship with the stator and defining an air gap with the stator, said rotating magnetic flux having a particular distribution in said air gap and exhibiting a predetermined number of angularly displaced radial polarity centers, and an active component included in the rotor and having an annular configuration and a cross-sectional area which diminishes from a maximum at certain angular positions on the rotor to a minimum at other angular positions on the rotor, said certain angular positions on the rotor being disposed between the radial polarities of the rotating magnetic flux during synchronous operation of the motor and said other angular positions being substantially in angular alignment with said radial polarities during such synchronous operation, said active rotor component being composed of hysteresis magnetic material of a particular coercivity and presenting an essentially uniform flux density path to the rotating flux in said stator bore during synchronous operation of the motor.

3. In an alternating current synchronous motor of the hysteresis type, the combination of: a stator structure having a bore and including a stator winding for producing a rotating magnetic flux in said bore, a rotor rotatably mounted within the bore of the stator in concentric relationship with the stator and defining an annular air gap with the stator, said rotating magnetic flux having an essentially uniform distribution in the air gap and exhibiting a predetermined number of angularly displaced radial polarity centers, and an active component included in the rotor and having an annular configuration and a varying cross-sectional area which diminishes in a substantially linear relationship with respect to different angular positions on the rotor from a maximum at certain angular positions on the rotor to a minimum at other angular positions on the rotor, said certain angular positions on the rotor being disposed between the radial polarity centers of the rotating flux during synchronous operation of the motor, and said other angular positions on the rotor being in substantial angular alignment with such radial polarity centers at such synchronous operation, said active rotor component being composed of hysteresis magnetic material of a particular coercivity and presenting an essentially uniform flux density path to the rotating flux in the stator bore during synchronous operation of the motor.

4. In an `alternating current synchronous motor of the hysteresis type, the combination of: a stator assembly having a bore and including a stator winding for producing a rotating magnetic ux in said bore, a rotor rotatably mounted within the bore of the stator in concentric relationship with the stator to define an annular air gap therebetween, said rotating magnetic flux exhibiting a predetermined number of angularly displaced radial polarity centers and having an essentially sinusoidal distribution in the air gap between such polarity centers, and an active component included in `the rotor and having an annular convfiguration and a varying cross-sectional area which diminishes in a substantially sinusoidal relationship with respect to different angular positions on the rotor from a maximum at certain angular positions on the rotor to a minimum at other angular positions on the rotor, said certain angular positions on the rotor being disposed between the radial polarity centers of the rotating flux during synchronous operation of the motor `and said other angular positions on the rotor being in substantial angular alignment with such radial polarity centers during such synchronous operation, said active rotor components being composed of hysteresis magnetic material of a particular coercivity and presenting an essentially uniform ux density path to the rotating flux in the stator bore during synchronous operation of the motor.

5. In an alternating current synchronous motor of the hysteresis type, the combination of: a stator assembly having a bore and including a stator winding for producing a rotating magnetic flux in said bore, a rotor rotatably mounted within the bore of the stator in concentric relationship with the stator and defining an annular air gap with the stator, said rotating magnetic flux having a particular distribution in said annular air gap and exhibiting a predetermined number of angularly displaced radial polarities, and an active member forming an outer component of the rotor and having an annular configuration with a cylindrical-shaped outer surface to define said annular air gap with the stator, said active member having an inner surface shaped to define a varying crosssectional area for the active member, and said active member being composed of hysteresis magnetic material of a particular coercivity and presenting an essentially uniform flux density path to the rotating flux in the stator bore during synchronous operation of the motor.

6. The combination defined in claim 5 in which said varying cross-sectional area of said active member diminishes with respect to different angular positions on the rotor from a maximum at certain angular positions on the rotor to a minimum at other angular positions on the rotor, said angular positions on the rotor being disposed between the radial polarities of the rotating flux at synchronous operation of the motor and said other angular positions on the rotor being in substantial angular alignment with such radial polarities during such synchronous operation.

7. In combination for use with alternating current apparatus of the hysteresis type having a rotating magnetic flux, a rotor structure including,

an active component of hysteresis magnetic material of a particular coercivity having an annular configuration and of a varying cross-sectional area to present a path of essentially uniform flux density to the rotating flux during operation of the alternating current apparatus.

8. In combination for use with an alternating current synchronous motor of the hysteresis type having a rotating magnetic flux exhibiting a predetermined number of angularly displaced radial polarity centers, a rotor structure including,

an active component having an annular configuration and a cross-sectional area which diminishes from a maximum at certain angular positions on the rotor to a minimum at other angular positions on the rotor with the certain angular positions on the rotor disposed between the radial polarities of the rotating magnetic flux during synchronous operation of the motor and the other angular positions substantially in angular alignment with the radial polarities during synchronous operation and having the active rotor component composed of hysteresis magnetic material of a particular coercivity and presenting an essentially uniform` flux density path to the rotating flux during synchronous operation of the motor.

9. In combination for use with an alternating current synchronous motor of the hysteresis type having a rotating magnetic flux exhibiting a predetermined number of angularly displaced radial polarity centers, a rotor structure including,

an active component having an annular configuration and a varying cross-sectional area which diminishes in a substantially linear relationship with respect to different angular positions on the rotor from a maximum at certain angular positions on the rotor to a minimum at other angular positions on the rotor with certain angular positions on the rotor disposed between the radial polarity centers of the rotating flux `during synchronous operation of the motor and the other angular positions on the rotor in substantial angular alignment with the radial polarity centers at synchronous operation of the motor and having the active rotor component composed of hysteresis magnetic material of a particular coercivity and presenting an essentially uniform flux ydensity path to the rotating ux during synchronous operation of the motor.

llO. In combination for use with an alternating current synchronous motor of the hysteresis type having a rotating magnetic ux exhibiting a predetermined number of angularly displaced radial polarity centers with an essentially sinusoidal distribution between the polarity centers, a rotor structure including,

an active `component composed `of hysteresis magnetic material of a particular coercivity having an annular configuration and a varying cross-sectional area which diminishes in a substantially sinusoidal relationship with respect to diierent angular positions on the rotor from a maximum at certain angular positions on the rotor to a minimum at other angular positions on the rotor with the certain angular positions on the rotor disposed between the radial polarity centers of the rotating tlux during synchronous operation of the -motor and the other angular positions on the rotor in substantial angular alignment with the radial polarity centers during synchronous operation of the motor to present an essentially uniform flux density path to the rotating flux during synchronous operation of the motor. 11. In combination for use with an alternating current synchronous motor of the hysteresis type having a rotating magnetic ux exhibiting a predetermined number of angularly displaced radial polarities, a motor assembly, including,

an active member composed of hysteresis magnetic material of `a particular coercivity forming an outer component of the rotor and having an annular conguration with a cylindrical-shaped outer surface yand an inner surface with a varying cross-sectional area to present a uniform ux density path to the rotation liux during synchronous operation of the motor. l2. The combination defined in claim 11 wherein the varying cross-sectional area of the active member diminishes with respect to different angular positions on the rotor from a maximum at certain `angular positions on the rotor to a minimum at other angular positions on the rotor with the certain angular positions on the motor disposed between the radial polarities of the rotating ux at synchronous operation of the motor and the other angular positions on the rotor in substantial angular alignment Awith the radial polarities during synchronous operation.

References Cited in the tile of this patent UNITED STATES PATENTS FOREIGN PATENTS Great Britain Nov. 27, 1957 

